The present invention relates to a method of and circuit for digitally demodulating an amplitude-modulated signal, produced by suppressed-carrier amplitude modulation of two carriers of the same frequencies in a phase-quadrature relationship, provided with a circuit for determining the square a.sup.2 of the peak amplitude a of a sinusoidal reference signal which is in synchronism with the modulated signal and with sampling means; it also relates to the use of such a method in, for example, an embodiment of a demodulation stage for the chrominance signal of a video-frequency signal of a television receiver and actually to any television receiver comprising such a demodulation stage.
The French Patent Application No. 8218254 filed by Applicants on Oct. 29th, 1982, describes a demodulation circuit in which measures are taken to recover, from the modulated signal, on the one hand, a first digital signal which is proportional to the frequency of the modulated signal, and, on the other hand, a second digital signal which is proportional to the square of the peak amplitude of the modulated signal.
It is necessary to describe briefly the demodulation procedure used to determine the frequency and the square of the peak amplitude of the modulated signal described in said patent application.
If the general expression of the analog input signal is of the type x=a.multidot.sin .omega.t, the digital signals at the outputs of the first and second registers 20, 21 and of the analog-to-digital converter 10 have the respective expressions, at a constant sampling frequency (Fe=1/T): EQU x.sub.n =a.multidot.sin .omega.t.sub.n for t=t.sub.n ( 1) EQU x.sub.n+1 =a.multidot.sin (.omega.t.sub.n +.phi.) for t=t.sub.n +T (2) EQU x.sub.n+2 =a.multidot.sin (.omega.t.sub.n +2 .phi.) for t=t.sub.n +2T (3)
From these three consecutive measurements x.sub.n, x.sub.n+1, x.sub.n+2, it is possible to express the cosine of the angular phase shift .phi. by: ##EQU2##
As it is obvious that the phase shift .phi. is equal to ##EQU3## wherein Fe is the chosen, constant sampling frequency, Fs is the frequency of the modulated signal ##EQU4## it is obtained that: ##EQU5## which renders it possible to determine the frequency of the input signal and, for example, to demodulate the chrominance signal, which, for the SECAM system, is frequency-modulated.
In a similar way, it is possible to express, from three identical consecutive measurements, the square of the amplitude a of the signal by the following expressions, which can be easily derived from the expressions (1) to (3) by simple trigonometrical manipulations: ##EQU6## This can be expressed by the general formula: ##EQU7##
On the other hand, French Patent Specification No. 2,502,423, which corresponds to the French Patent Application No. 815,285, on Mar. 17th 1981, describes a demodulator for demodulating digital frequency-modulated or amplitude-modulated chrominance signals obtained by amplitude modulation with suppressed carrier of two quadrature-phase carriers. The specification describes a demodulation method based on a representation of the frequency-modulated or amplitude-modulated signal by a vector turning in a plane OXY, said signal being sampled by a signal having a frequency 4F, that is to say four times higher than:
either the center frequency of the frequency band for the case of frequency modulation (SECAM);
or the carrier frequency in the case of amplitude modulation (PAL).
For the case of amplitude modulation of the PAL type, which is of interest for the present case, the demodulation procedure is limited to sampling the modulated signal by a signal having a frequency 4F, the case of amplitude modulation being converted to that of frequency modulation where simplifications become apparent connected with approximations relative to the frequency deviations.